The present invention relates to the compression of digital video, and more particularly to a method and apparatus for processing digitized interlaced video signals for transmission in a compressed form.
Television signals are conventionally transmitted in analog form according to various standards adopted by particular countries. For example, the United States has adopted the standards of the National Television System Committee ("NTSC") while most European countries have adopted either PAL (Phase Alternating Line) or SECAM standards.
Digital transmission of television signals can deliver video and audio services of much higher quality than analog techniques. Digital transmission schemes are particularly advantageous for signals that are broadcast by satellite to cable television affiliates and/or directly to home satellite television receivers. It is expected that digital television transmitter and receiver systems will replace existing analog systems just as digital compact discs have largely replaced analog phonograph records in the audio industry.
A substantial amount of digital data must be transmitted in any digital television system. This is particularly true where high definition television ("HDTV") is provided. In a digital television system where signals are transmitted by satellite, the television signals can be transmitted using a quadrature phase shift keyed ("QPSK") modulated data stream. A subscriber to the system receives the QPSK data stream via a receiver/descrambler which provides video, audio, and data to the subscriber. In order to most efficiently use the available radio frequency spectrum, it is advantageous to compress the digital television signals to minimize the amount of data that must be transmitted.
Video compression techniques have been used in the past for video teleconferencing and other specialized applications. Such systems are capable of very high compression ratios, but generally exhibit limited spatial resolution and poor motion rendition. This is usually a result of initial constraints imposed on the frame rate and on the horizontal and vertical sampling rates of the system. A video "frame" can be likened to one of a sequence of snapshots that together provide a moving picture. Each frame is sampled in both the horizontal and vertical direction to obtain all of the picture information contained therein.
Video compression systems are currently under development for digital transmission of existing television signals and future high definition television signals. Such television signals are significantly more complex than teleconferencing signals and are much more difficult to compress. The performance of digital compression systems in television applications is highly scene dependent. In order to succeed, a compression algorithm should be able to adapt to specific conditions to increase compressibility and to mask invariable errors in a manner that will not be perceivable to a human viewer.
Highly detailed moving objects in a television picture present the greatest challenge to a compression system. The most powerful compression systems currently available (i.e., those that achieve the greatest reduction in the amount of data necessary to define television pictures) utilize interframe processing in order to take advantage of the temporal correlation between successive frames. However, frame-to-frame correlation is reduced when there is movement. This requires more complicated processing to maintain a high degree of performance.
Video compression is further complicated with television signals since interlaced scanning is used to define a television picture. Each frame of a television picture comprises a plurality of horizontal lines (e.g., 525 lines in a standard NTSC television signal) which together form a picture. The horizontal lines are divided into even and odd fields, wherein the even lines (lines 2, 4, 6, . . . ) form the even field and the odd lines (lines 1, 3, 5, . . . ) form the odd field. The even and odd fields are scanned in an alternating order to interleave the even and odd lines and provide the picture information in a proper sequence. The use of interlaced scanning complicates the compression of television signals as compared to previous teleconferencing applications, in which the compression was not performed on an interlaced signal.
A digitized interlaced television signal can be compressed in various formats. In one format, referred to herein as the "field format", each frame is separated into its two fields which are processed independently. In another format, referred to as the "frame format", the two fields are processed as a single frame by interleaving the lines of corresponding even and odd fields. Neither option is entirely satisfactory for video compression. Frame processing works better than field processing when there is little or no motion. Since each frame has twice the number of lines or samples than a field for a given picture height, there will be more correlation between samples and hence, compressibility is increased. To achieve the same accuracy as frame processing, field processing requires a higher bit rate. Thus, for equal bit rates frame processing achieves greater accuracy.
Frame processing enjoys similar advantages over field processing if horizontally moving features have little horizontal detail or if vertically moving features have little vertical detail. In regions where there is little detail of any sort, frame processing often works better than field processing no matter how rapidly changes occur.
In detailed moving areas, it is generally more efficient to compress field formatted data. In such cases, frame processing suffers from spurious high vertical frequencies introduced by the interleaving of the even and odd fields. This reduces the correlation between lines and therefore the effectiveness of the compression algorithm.
It would be advantageous to provide a compression system that combines the advantages of frame processing where there is little or no motion with the advantages of field processing in detailed moving areas. It would be further advantageous to provide such a system that permits video signals to be compressed and then reconstructed without any degradation in motion rendition.
It would also be advantageous to provide a general purpose compression system for optimizing the compression of digital data by combining different compression techniques or data formats to obtain peak performance under different conditions. The present invention provides a method and apparatus for achieving such advantages.